The other reason and one of the most common ones is the dishonesty and they are different things like if you cheat on your girlfriend
or boyfriend and then they find out you will broke up and start hating on each other and sometimes he or she will start insulting you and all that. there are many causes like a fake friend(a fake friend is someone that acts like your friend) and if you are a fake friend you probably start beating up each other or you humiliate him in public and all that and the only thing you can do is be a good friend or be loyal to your girlfriend or boyfriend.
this is the most common problem is disagree with each other and do not think that you friend has a good idea and you start complaining and saying that his idea is bad and you just want to do it your way and then you start insulting his idea and then you just care what he will be doing. The only thing that i would say is that you should try that idea and if it does not work do it your way and if you do not want to do that start thinking ways you can agree.